At this stage in mammals the blastula is called the blastocyst, which consists of an outer epithelium, the trophectoderm, enveloping the inner cell mass and the blastocoel .

In a process called cavitation, trophectoderm cells transport fluid into the embryo to create a blastocoel, the fluid-filled lumen.

The oviduct cells stimulate these trophoblast sodium pumps as the fertilized egg travels down the fallopian tube towards the uterus.

It was demonstrated in the frog embryo that the first cleavage furrow widens in the animal hemisphere creating a small intercellular cavity that is sealed off via tight junctions.

[4] During the next stage of embryonic development, amphibian gastrulation, the blastocoel is displaced by the formation of the archenteron, during mid-gastrulation.

[11] Even as the blastomeres continue to divide, the blastula remains one-cell thick and thins out as the embryo expands outward.

This is accomplished in part due to the influx of water that expands the blastocoel and pushes the cells surrounding it outwards.

From there the blastodisc, a small cluster of cells in the animal pole of the egg, then undergoes discoidal meroblastic cleavage.

The shape and formation of the avian blastodisc differs from amphibian, fish, and echinoderm blastulas, but the overall spatial relationship of the blastocoel remains the same.

[5][page needed] Unlike amphibian, echinoderm, mammalian, and avian embryos, zebrafish do not have a defined blastocoel.